This application is based on and claims the benefit of German Patent Application No. 10051962.8 filed Oct. 20, 2000, which is incorporated by reference herein.
The present invention relates to an insulated electrical conductor which will continue to function in case of fire, an electrical cable incorporating such a conductor, and a process for producing an insulated electrical conductor and an electrical cable.
When cables are exposed to flames, the insulation and sheath materials that are present in the cable usually burn, unless these materials have already completely or partially melted away due to the heat of the fire. Any residues remaining on the conductors after the fire, unless they have become conductive through carbonization or through the action of extinguishing agents, may prevent a short circuit between the conductors or a ground fault and thus permit emergency operation at low operating voltages. As a rule, however, these residues are unable to withstand mechanical loads, so that even the slightest movements, such as thermally induced changes in the length of the conductors during cooling after the fire, or slight vibrations cause the residues to be destroyed and the cable to fail. Cables that must remain operational in case of fire, for instance cables for emergency call systems or for the operation of fire extinguishing systems, are insulated with materials that are stable in a fire. Additionally inserted mineral layersxe2x80x94such as continuous glass filament stripsxe2x80x94can maintain the insulating properties in a fire. Here, too, however, the insulation capacity may be significantly affected by extinguishing agents.
The greatest safety with respect to operability in case of fire is achieved with mineral-insulated cables. These cables are insulated with a solid ceramic mass and enclosed with a sheath of metal. Such cables are extremely expensive, however, and have little flexibility.
U.S. Pat. No. 3,425,865 discloses an insulated conductor in which a first coating of an inorganic barrier material is applied to the conductor. This is a strip of glass fabric reinforced with mica. The mica particles are bonded to the glass fabric strip by means of a silicon resin. The layer may be applied to the conductor by extrusion, in the form of a tape, or in some other manner. In addition, a second abrasion resistant polyimide layer is placed over the first layer. Additional insulation layers may be applied to this polyimide layer. An insulated conductor produced in this manner is distinguished by its low weight, high abrasion resistance, and high flame resistance.
DE Utility Model 87 16 166 discloses a heat resistant electrical cable having a nickel-coated copper conductor, which is sheathed by at least one layer of mica tape and a glass filament braid placed on top of this layer. A metal strip wound with overlaps is applied on top of the glass filament braid and a braid made of metal wires on top thereof. Such a construction is very costly and at best marketable for special applications. The cable furthermore has little flexibility and is very heavy due to the high metal content.
The object of the present invention is to provide an insulated electrical conductor which is flexible, has little weight and can be produced cost-effectively.
This object is attained by an insulated electrical conductor comprising a metallic conductor, a first glass and/or mica containing layer applied to the conductor, and a second plastic layer sheathing the first layer, characterized in that the first layer is made of at least two longitudinally introduced strips (2, 3) of glass and/or mica, which are applied to the conductor, wherein the width of said strips (2, 3) is selected such that the strips (2, 3) overlap each other by at least 50%.
The object of the invention is further attained by a cable including at least two conductors of the type described above.
The essential advantage of the invention is that the longitudinal introduction of, e.g., two mica containing strips and the required overlap actually produces three mica layers on the conductor. If a winding with a thread or strip of a high tensile, flame resistant material is provided, the two strips are held together during production as well as in case of fire.
Other advantages of the invention will be apparent from the description and claims below.